The correct calculation is not 900€/36 months but (900€-$resell_value)/36 months. If you sell your GPU for 450€ after three years you saved a good bit of money. If the AI bubble doesn't pop, your resale value might even be a good bit higher than that. I've had a used 1080TI that I used for five years and then sold for nearly the same price, making it effectively free (minus electricity use and opportunity cost)

Even if you don't resell it, at the end of the three years you still have a GPU that you can keep using, or gift, or whatever. After three years of renting, you have nothing.